Phthalocyanine Langmuir-Blodgett Films Research Articles

Supramolecular architectures of iron phthalocyanine Langmuir-Blodgett films: The role played by the solution solvents

The developing of organic-based devices has been widely explored using ultrathin films as the transducer element, whose supramolecular architecture plays a central role in the device performance. Here, Langmuir and Langmuir-Blodgett (LB) ultrathin films were fabricated from iron phthalocyanine (FePc) solutions in chloroform (CHCl3), dichloromethane (CH2Cl2), dimethylformamide (DMF), and tetrahydrofuran (THF) to determine the influence of different solvents on the supramolecular architecture of the ultrathin films. The UV–vis absorption spectroscopy shows a strong dependence of the FePc aggregation on these solvents. As a consequence, the surface pressure vs. mean molecular area (π-A) isotherms and Brewster angle microscopy (BAM) reveal a more homogeneous (surface morphology) Langmuir film at the air/water interface for FePc in DMF. The same morphological pattern observed for the Langmuir films is preserved upon LB deposition onto solid substrates. The Raman and FTIR analyses indicate the DMF-FePc interaction relies on coordination bonds between N atom (from DMF) and Fe atom (from FePc). Besides, the FePc molecular organization was also found to be affected by the DMF-FePc chemical interaction. It is interesting to note that, if the DMF-FePc leads to less aggregated FePc either in solution or ultrathin films (Langmuir and LB), with time (one week) the opposite trend is found. Taking into account the N-Fe interaction, the performance of the FePc ultrathin films with distinct supramolecular architectures composing sensing units was explored as proof-of-principle in the detection of trace amounts of atrazine herbicide in water using impedance spectroscopy. Further statistical and computational analysis reveal not only the role played by FePc supramolecular architecture but also the sensitivity of the system to detect atrazine solutions down to 10−10mol/L, which is sufficient to monitor the quality of drinking water even according to the most stringent international regulations.

Annealing assisted structural and surface morphological changes in Langmuir–Blodgett films of nickel octabutoxy phthalocyanine

We report our studies on thin films of metallo-phthalocyanine (MPc), Nickel(II)1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (NiPc(OBu)8) transferred in a well defined thermodynamic state over a self assembled monolayer (octadecyl trichlorosilane)/SiO2/Si substrate using the Langmuir–Blodgett (LB) method. The films are characterized using differential scanning calorimetry (DSC), grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM) techniques. DSC studies on powdered samples in the bulk indicate enantiotropic solid–solid phase transition. GIXD studies on the as-deposited LB film show a Bragg peak indicating crystallinity of the thin film. Annealing (373K) results in reduction of lattice spacing (1.21Å) signifying changes in molecular packing within the unit cell. At this stage, an additional Bragg peak is observed which grows at the expense of the former one and they coexist between 373K and 423K. A discontinuity in lattice spacing from 20.73 to 15.12Å with annealing indicates clearly a structural change of the underlying crystalline lattice. Correspondingly, the surface morphology images obtained using AFM show, with annealing, a transformation from spherical granular morphology to elongated, flat crystallites suggesting asymmetric growth process. Statistical parameters of the grain extracted from the AFM images show that the size, fractal dimension and circularity are affected by annealing. Based on these studies, we infer the structural and surface morphological changes of the meta-stable phase (Form I) to the stable phase (Form II) in annealed LB films of phthalocyanine.

Investigation of the Adsorption Behavior of PbPc on Graphene by Raman Spectroscopy

Using graphene-enhanced Raman scattering, the Raman signals of molecules attached to graphene can be obtained. For different molecules and vibrational modes, the enhancement factors are different. Here, we have investigated the variation in the adsorption behavior of lead phthalocyanine (PbPc) Langmuir-Blodgett (LB) films on graphene under annealing using Raman spectroscopy. With increasing annealing temperature, it was found that the Raman intensity of the PbPc molecules first increased and then decreased. At the sublimation temperature, the enhanced Raman signal was the strongest, indicating that the orientation of the PbPc molecules had changed from perpendicular to parallel to the graphene surface. As the annealing temperature was increased towards the sublimation temperature, some vibrational modes with low Raman scattering cross-section appeared, and they were enhanced at higher temperatures. This indicates that the PbPc molecules are deformed due to π-π interactions with graphene, and change their structure from nonplanar to planar. When the annealing temperature was increased even further, some new vibrational modes appeared, which can be attributed to the reduction of Pb(II) to Pb(0) in the PbPc molecules.

Percolation and transport properties of tetrasubstituted manganese phthalocyanine Langmuir–Blodgett films in the inhomogeneous lateral electric field

The results of electric conductivity study for the tetrasubstituted manganese phthalocyanine Langmuir–Blodgett films (R C(CH 3) 3) placed in lateral inhomogeneous electric field are reported. The inhomogeneous electric field is provided by the lateral surfaces of silicon p-n junctions where the phthalocyanine films are deposited on. It is shown that the lateral conductivity of the film affected by the inhomogeneous field is 10 6 times higher than in the case of the homogeneous field. The anomalous behavior of the conductivity relaxation is observed. The model of interacting sticks, which explains experimental results, is proposed. Computer simulation of the conductivity relaxation, percolation threshold and resistivity of sticks system are carried out using the proposed model. The results of the simulation are in agreement with observed experimental dependencies.

Effects of laser irradiation and annealing on rhodium phthalocyanine Langmuir–Blodgett films

The effects of laser irradiation and annealing of rhodium phthalocyanine Langmuir–Blodgett films deposited on quartz and Au-covered glass substrates were studied with ultraviolet-visible and Fourier transform infrared spectroscopy, X-ray diffraction and atomic force microscopy. We found that laser irradiation and annealing produce very similar results attributed to the heating effect in both cases and produce different results in different atmospheres (air and Helium). Changes observed in films treated at 200 °C in an inert atmosphere are attributed to a molecular rearrangement of the films associated with the destruction of J aggregates. Films treated in similar conditions in air show, besides this effect, possible oxidation and loss of molecules. A remarkable decrease of roughness was observed when the films are annealed at 100 °C in He, attributed to the evaporation of water molecules trapped in the Langmuir–Blodgett films.

Polar Properties of Langmuir–Blodgett Films of Copper Phthalocyanines

Langmuir-Blodgett films of amphiphilic copper phthalocyanines are obtained. The Stark effect spectra and the spectra of pyro-and photosensitivity of these films are investigated. The following molecular parameters are determined: changes in the polarizability and the dipole moment for individual absorption bands in the Langmuir-Blodgett films and in an isotropic polymer solution. It is shown that the films obtained have photosensitivity due to the charge transfer in the spectral range 600–900 nm. The pyroelectric effect was observed, which indicates that the films have a polar structure.

Field-Effect Transistors Based on Langmuir−Blodgett Films of Phthalocyanine Derivatives as Semiconductor Layers

Two soluble phthalocyanine derivatives, 2,9,16-tri(tert-butyl)-23-(10-hydroxydecyloxy) phthalocyanine (1) and 2,9,16-tri(tert-butyl)-23-(10-hydroxydecyloxy) copper phthalocyanine (2) were used as semiconductor layers in organic thin-film transistors (OTFTs) based on their Langmuir−Blodgett (LB) films. From the π−A curves, we can conclude that the phthalocyanine 1 and 2 molecules are in a titled arrangement at the air−water interface. The molecular ordering in the LB multilayer films on the SiO2 substrate was examined by using UV−Vis absorption spectra and atomic force microscopy (AFM). The highest occupied molecular orbital and the lowest unoccupied molecular orbital were determined, and the energy band diagram of their OTFTs can be deduced by the cyclic voltammetric measurements. The LB films of phthalocyanine 1 and 2 were subsequently made into p-channel field-effect transistors, which were generally operated in the enhanced mode. The channel mobilities of phthalocyanine 1 and 2 were calculated to be about 4.8 × 10-5 cm2 V-1 s-1 and 4.0 × 10-4 cm2 V-1 s-1, respectively.

Second harmonic generation at the interface of copper tetra-tert-butyl phthalocyanine Langmuir-Blodgett film/metal

Optical second harmonic generation (SHG) from the copper tetra-tert-butyl phthalocyanine (CuttbPc) Langmuir-Blodgett (LB) film deposited on a metal-coated glass slide substrate has been investigated. It is considered that the symmetry of the CuttbPc molecule may be broken by the space charge-induced electric field (SCIEF) due to the exchanged charges at the CuttbPc LB film/metal interface. A four-layer model is used to explain the nonlinear optical process in the CuttbPc LB film. The thickness dependence, polarized and incident angle dependence of SHG signal from CuttbPc LB films are calculated. The results of calculation show a good agreement with the experimental data. It is shown that electrostatic phenomena at the interface is correlated closely with SHG signal, and the SHG measurement is also a helpful tool for the detection of the space charge field at the interface.

Spectroscopic Properties of Langmuir−Blodgett Films of Lanthanide Bis(phthalocyanine)s Exposed to Volatile Organic Compounds. Sensing Applications

The possibility of using Langmuir−Blodgett (LB) films of bis(phthalocyaninate) rare earth compounds (LnPc2) (including praseodymium, gadolinium, and lutetium bis(phthalocyanine)s and their octa-tert-butyl derivatives) as optical sensors has been explored. Special attention has been paid to the previously unexplored near-infrared (NIR) region. First of all, the NIR spectra of the unexposed LB films have been recorded. The NIR spectra of the bis(phthalocyanine) LB films are reported here for the first time. The changes in the UV−vis−near-infrared spectra caused by the interaction with several gases including NOx and vapors of volatile organic compounds (VOCs) such as alcohols, aldehydes, and esters (which are mainly responsible for odors in foods and beverages) have been studied. The influences of the response of the central metal atom of the phthalocyanine molecules and of the presence of tert-butyl groups in the peripheral benzene rings have been evaluated. It has been evidenced that the changes that take place in the NIR region occur at telecommunications wavelengths and open the possibility of using the bis(phthalocyanine) LB films as the sensitive part in industrial fiber optic sensors.

Interfacial electrostatic phenomena in phthalocyanine Langmuir–Blodgett films

Abstract Surface potentials of Cu-tetra- tert -butyl-phthalocyanine (CuttbPc) Langmuir–Blodgett (LB) films on metal electrodes were measured under photoillumination in a vacuum vessel. The surface potential of CuttbPc LB films on Al and Cr electrodes, positively charged as deposited. Decreases very little but not zero by photoillumination, whereas the potential of films on Al and Cr electrodes increases. A linear relationship between the work function of metal electrodes and the saturated surface potential of CuttbPc LB films was obtained in dark and under photoillumination, indicating that electronic charges are exchanged at the interface between metal electrodes and film. The space charge distribution in CuttbPc LB films was determined on the order of nanometer film-thickness scale. It was concluded that about 1% of CuttbPc molecules make a contribution to the charge exchange and 9–27% of the contributed charges lose the charge exchange by photoillumination.

Imidisation of Langmuir–Blodgett films using a supercritical medium

Abstract Chemical imidisation of Langmuir–Blodgett (LB) films of polyamic acid- n -octadecyl amine salt was performed in the presence of carbon dioxide at supercritical conditions. The supercritical medium constituted a replacement for benzene as the solvent vehicle for transporting the dehydrating agent (acetic anhydride) and catalyst (pyridine) to the polyimide precursor. FTIR spectroscopy showed that imidisation was complete and that the spectral characteristics were similar to that of films imidised by conventional thermal and chemical methods. However, the reaction is completed in a much shorter time than that required for imidisation in benzene. Use of the supercritical medium facilitated flushing of the reaction vessel and ensured complete removal of the unused reagents and reaction products, thereby obviating the need for post-imidisation rinse and drying. The application of this method in preparing composite LB films containing organic guest species is demonstrated by the fabrication of functional polyimide–copper butoxy phthalocyanine and composite LB films. Thermal imidisation of the composites at temperatures above 250 °C and chemical imidisation in benzene resulted in loss of the guest species, possibly due to evaporation and dissolution, respectively. On the other hand, the guest molecules were retained in the LB films that were chemically imidised in supercritical CO 2 . Atomic force microscopy showed that phthalocyanine tended to form crystalline aggregates in the thermally imidised LB film and not in the CO 2 -imidised one. XPS investigations did not show the presence of copper in the CO 2 -imidised film, suggesting that the chromophore may be present in a diluted/de-aggregated form on the surface.

Influence of transfer promoters on the deposition and wettability characteristics of copper tetra-tert-butyl phthalocyanine Langmuir–Blodgett films

This study investigated the influence of three transfer promoters on the deposition and wettability characteristics of copper tetra-tert-butyl phthalocyanine (CuTTBPc) Langmuir–Blodgett (L-B) films. With the addition of transfer promoters, such as stearyl alcohol, stearamide, and arachidic acid, two transitions corresponding to monolayer collapse were observed in the surface pressure-area isotherms of mixed CuTTBPc/transfer promoter monolayers at the air/water interface. Nevertheless, the expected enhanced stability of a CuTTBPc monolayer in the presence of transfer promoters did not reflect in the monolayer area relaxation curves obtained at constant surface pressures. The transfer ratio for the deposition of a CuTTBPc L-B film on a glass substrate was close to one, and thus the addition of transfer promoters showed little improvement in the deposition. On a gold-coated glass substrate, however, the deposition of a CuTTBPc L-B film was considerably improved by the existence of arachidic acid. Surprisingly, the deposition was not improved with the addition of stearyl alcohol or stearamide. Moreover, the presence of transfer promoters did not show significant influence on the wettability characteristics of the mixed CuTTBPc/transfer promoter L-B films on glass or gold-coated glass substrates except for the case of arachidic acid.

Enhancement of second-harmonic generation for phthalocyanine Langmuir–Blodgett films on metal electrodes

Optical second-harmonic generation (SHG) from LB films of phthalocyanine derivative deposited on metal (aluminum: Al) and glass substrate has been investigated. In spite of the existence of the inversion center, for both Al and glass substrates, the bulk characteristic of the SHG was confirmed by the thickness dependence of the SH intensity. Drastic enhancement of the signal intensity for Pc/Al sample was also observed. In order to clarify the enhancement mechanism, the thickness-dependent SH intensity was calculated in terms of the quadrupole mechanism. As a result, it is possible that the Al layer enhances the SHG due to the quadrupole process.

Peculiarities of optical properties of rare earth elements phthalocyanine LB films

Optical properties of tert-butyl-bis-Pc of Ho, Dy, Sm in different aggregate states — solution, Langmuir–Blodgett (LB) films and vacuum-deposited (VD) films have been studied. The shift of absorbance maxima in LB and VD films, which are only 100–230 cm−1 compared to hexane solution, can be explained by weak intermolecular interaction. Thus, it is shown that LB films of H[(4tPc)2Ho] are enriched by red (positive charged) form and VD ones by both charged with predominance of blue (negative charged) one. The emergence of blue form in VD films can be explained by difference of sublimation temperatures, and this is confirmed by decrease of the blue form content with thickness increase. Besides three new bands were found with maxima near 23 500, 25 400 and 27 000 cm−1. Their formation is attributed mainly to red, blue and green form, respectively. The effect of rare earth elements (REE) atom on parameters absorbance is discussed.

Nanometric electrostatic interfacial phenomena in organic semiconducting thin films

Surface potentials of two kinds of phthalocyanine Langmuir–Blodgett (LB) films deposited on metal electrodes were examined as a function of the number of deposited layers. It was found that the potential built across the LB films was due to the displacement of excessive electronic charges (electrons and holes) from metals. The spatial charge distribution in these LB films was determined to be of the order of nanometres in film thickness, and the distribution of the electronic density of states was also determined assuming the presence of both electron donating and accepting states at the metal/film interface. The capacitance–voltage (C–V) and current–voltage (I–V) characteristics of phthalocyanine LB films sandwiched between metal electrodes were examined taking into account the interfacial electrostatic phenomena.

Interfacial electronic density of states in phthalocyanine derivative Langmuir–Blodgett films determined by surface potential measurement

Surface potentials of phthalocyanine derivative Langmuir–Blodgett (LB) films on metal electrodes were studied. Two kinds of phthalocyanine derivatives (C6PcCu and CuttbPc) were examined as a function of the number of deposited layers. The distributions of spatial excess charge density and electronic density of states at the metal/film interface were determined. The difference in the electrostatic phenomena observed in these two phthalocyanine LB films was attributed to the difference in the density of surface states of these two films.

Interfacial Electronic Phenomena and Capacitance–Voltage Characteristics of Phthalocyanine Langmuir-Blodgett Films

The capacitance–voltage (C–V) characteristics of ultrathin phthalocyanine (CuttbPc) Langmuir-Blodgett (LB) films were examined, taking into account the interfacial electrostatic phenomena. It was revealed that the electrical properties of CuttbPc LB film sandwiched between metal electrodes were dominated by negatively charged space-charge layers with a thickness of less than 10 nm. It was also shown that the density of charges in the electrostatic double layer of Al/CuttbPc LB film, determined from a low-frequency C–V measurement, is in good agreement with the space-charge density, obtained by the conventional surface- potential measurement.

Electrical measurement on a phthalocyanine Langmuir-Blodgett film: II.The effect of the substrate surface treatment on the electricalcharacteristics

The stability and reproducibility of the electrical resistance measured on a thin Langmuir-Blodgett (LB) film of a copper tetraoctadecyltetrapyridino [3,4-b : 3´,4´-g : 3´´,4´´-l : 3´´´,4´´´-q] porphyrazinium bromide (CuS18) were improved with the control of the organic/inorganic interface between the film and the substrate. Homogenization of the physical surface properties both of the gold electrodes and of the glass substrate was obtained through grafting self-assembled monolayers onto both types of surfaces. Such a strategy allowed fabrication of samples with reproducible resistance values due to a better quality of the LB transfer of the organic monolayer onto the substrate. However, electrical drift with the elapsed time was still observed (a factor of 103 in 3 months). This ageing behaviour of the electrical resistance appeared closely related to the instability of the gold clusters located at the border of the electrodes. Their immobilization obtained with the treatment of the substrate with a thiolsiloxane resulted in a much slower and reproducible electrical drift.

金属/フタロシアニン系LB超薄膜界面の界面電子性準位と電気特性

金属/フタロシアニン系LB超薄膜界面の界面電子性準位と電気特性

Nano-electrostatic phenomena in Langmuir–Blodgett films

Surface potentials of heat-treated ultrathin polyimide (PI) Langmuir–Blodgett (LB) films and semiconducting phthalocyanine LB films deposited on Au, Cr and Al electrodes were measured by means of the Kelvin-probe method in a dark vacuum vessel at various temperatures as a function of the number of deposited layers. The spatial charge distribution in these LB films was determined on the order of nanometer scale from the relationship between the surface potential and the number of deposited layers. The distribution of the density of electronic surface states in these LB films was also determined. It was shown that the charge exchange phenomena at the metal/film interface are explained with taking into account the presence of the interfacial states within the range of about 1 nm from the metal/film interface. Finally we discuss the molecular-rectifying property of junctions using organic LB films sandwiched between Al and Au electrodes.